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ENERGY Energy is the ability to do work You do work when you use a force to move something. If you have a lot of energy, you can do a lot of work. Everything around us depends on energy Example: a gale force wind has more energy than a gentle breeze Anything that does work must have a supply of energy. Example: when you run, the energy comes from the muscles in your body. Your muscles get their energy from the food you eat When we talk about how much energy something has, it is important to have a unit for measuring energy. Energy has a unit called the joule. You use one joule to lift a 100 gram mass one metre. Because a joule is only a small amount of energy it is common to use kilojoules (kj) and megajoules (Mj) 1 kilojoule = 1000 joules 1 megajoule = 1000000 joules There are many different types of energy. Kinetic energy – any moving object has energy. When you run, you have kinetic energy. A moving train has a large amount of kinetic energy As a moving object slows down, it loses energy. When the object stops, it has no kinetic energy. The amount of kinetic energy an object has depends on its speed. The faster the object moves, the more kinetic energy it has. For example, a cricket ball bowled by a fast bowler has more kinetic energy than one bowled by a spin bowler Kinetic energy also depends on the mass of the moving object. The larger the mass, the greater its kinetic energy. For example, a cyclist and bus may be travelling at the same speed, but the bus has much more kinetic energy because it has greater mass. Much of the energy around us is stored energy. It has the potential to do work, so stored energy is called potential energy. For example, the stored energy something has when it is high is called gravitational potential energy. This energy is ready to be used because of the pull of gravity. When you are at the top of a slide, you have gravitational potential energy – you have the potential to slide to the bottom. The heavier you are, and the higher the slide, the more potential energy you have. As you slide down, this gravitational energy is changed to kinetic energy. What other objects might have gravitational potential energy? When you jump on a trampoline – what pushes you into the air. When you land on the mat, it moves down stretching the springs and storing energy called elastic potential energy. As the stretched springs return to their original size and shape, they release their stored energy. What other objects might have elastic potential energy? Energy is stored in chemicals as chemical energy. For example, when fuels such as wood and petrol are burned, this stored energy is released as heat and light. Foods also contain chemical energy that can be used by our bodies. What other examples can you think of that might have chemical energy? Sound is a form of kinetic energy caused by vibrating objects. It travels from place to place as sound waves. The louder the sound is, the more energy it has. Heat is form of energy that hot objects have. If heat energy is taken away from an object, it becomes cooler. Examples include refrigerators and airconditioners. Burning chemicals, very hot objects and stars all release light energy. It travels through space in waves Examples include: Radio and TV waves Light energy from the sun (solar energy), is used by plants to make their food. Energy: Review questions Complete the following questions 1. 2. 3. 4. 5. 6. What is energy? What unit of measurement is used to measure energy? Explain what kinetic energy is? Give an example What two factors determine the kinetic energy of an object? What is gravitational potential energy? Give two examples. Explain the difference between elastic potential energy and gravitational potential energy. 7. What forms of energy do the following have: 1. The water in a waterfall 2. A block of chocolate 3. A car travelling along a road 4. A child sitting on the top of a slide 5. A child bending a ruler 6. A wound up toy